WASHINGTON, July 13 (Reuters) - U.S. Attorney General Eric Holder said he is concerned fighters from Europe and the United States who are supporting violent insurgents in the Syrian civil war are joining forces with Yemeni bomb makers.

"In some ways, it's more frightening than anything I think I've seen as attorney general," Holder said on ABC's "This Week," broadcast on Sunday.

U.S. intelligence agencies estimate around 7,000 of the 23,000 violent extremists operating in Syria are foreign fighters, mostly from Europe.

Holder, who last week met with European justice ministers in London, said the worry is not only about the actions of foreign fighters in Syria, but when they return to their home countries.

Extremists have tried to recruit Westerners and send them back home with a mission. U.S. Homeland Security Secretary Jeh Johnson has said Syria has become a matter of homeland security.

The FBI has dozens of investigations under way on American fighters who have gone to Syria and made their way home, Holder said.

Intelligence that bomb makers in Yemen have been joining forces in Syria with the foreign fighters is particularly concerning, he added.

"That's a deadly combination, where you have people who have the technical know-how along with the people who have this kind of fervor to give their lives in support of a cause that is directed at the United States and directed at its allies," Holder said in the interview that was recorded last week.

A Nigerian man who attempted to detonate explosives on a flight from Amsterdam to Michigan in 2009, and who became known as the underwear bomber, was linked to an extremist group that operates in Yemen.

Last week, Holder urged countries in Europe and elsewhere to do more to keep their citizens from traveling to Syria to fight, saying they could learn from U.S. efforts to conduct undercover sting operations.

Holder said Sunni extremists flowing into Iraq from Syria who have seized towns in the North do not yet represent a threat to the West. But that could change.

"If they are able to consolidate their gains in that area, I think it's just a matter of time before they start looking at the West and at the United States in particular," he said.

Earlier this month, a Denver woman accused of trying to fly to Syria to support insurgents there was arrested in the United States, and last month two men in central Texas were arrested on similar charges.

One of the Texas men was charged with "attempting to provide material support to terrorists," violating a law that Holder urged other countries to copy. (Reporting by Timothy Gardner; Editing by Jim Loney and Cynthia Osterman)